Heh, I'd asked it to be defined in this order because of backwards compatibility :) If we look for `package.json` first, we can support the case where an add-on supports Firefox versions that don't have native Jetpack support (by including `install.rdf` and the relevant support files), but when installed in a version of Firefox that *does* have native Jetpack support it'll automatically get to use all the native stuff.

Looking at the add-ons MXR, there's only one add-on that ships with a `package.json` file in the root directory (AlwaysRememberPassword, which only has 134 people using it).